In a machine for processing mail articles, the invention relates to a method of distinguishing between mail articles contained in envelopes made out of paper and mail articles contained in envelopes made out of plastics material.
Some postal applications require bar codes to be printed on the envelopes of the postal items. In practice, the mail processed by sorting machines is packaged not only in envelopes made out of paper, but is often also packaged in envelopes made out of plastics material. Printing bar codes on envelopes made out of plastics material thus presents a problem since ink takes a long time to dry on that type of printing medium, and the printed bar codes are likely to be wiped off due to the envelopes rubbing against the sorting machines. Stickers serving as a medium for the bar codes to be printed can be put on items processed by a sorting machine, and in order to avoid putting stickers on all such items it has been envisaged to distinguish between envelopes made out of plastics material and envelopes made out of paper so that a sticker is put on an item only if it is contained in an envelope made out of plastics material.
Several techniques are already known for distinguishing between envelopes made out of paper and envelopes made out of plastics material. One known technique is based on an optical principle consisting in reflecting a beam of light onto the envelope to be inspected, and in measuring the reflected energy. Since plastics material is more reflective than paper, the distinction is simple to make. However, that technique of distinguishing by optical measurement can turn out to be ineffective when the envelopes made out of plastics material are matt and/or colored, i.e. when they have reflectance that is similar to that of paper. Another known technique is based on a pneumatic principle consisting in deforming the envelope to be inspected by suction, and in measuring a pressure variation associated with said deformation. Since an envelope made out of plastics material is generally less rigid than an envelope made out of paper, the distinction is simple to make. However, the effectiveness of that technique of distinguishing by pneumatic measurement is very sensitive to dust and therefore requires the pressure sensor to be cleaned frequently so as to maintain its measurement performance. U.S. Pat. No. 5,632,381 discloses a method of classifying sheets of paper and of card based on comparing the acoustic signal produced by tapping the sheets with a ram. Patent DE-4 217 481 proposes classifying objects made out of paper, aluminum, and plastics by electrostatic means.
The object of the invention is to propose another solution, which does not have the above-mentioned drawbacks, for distinguishing between mail articles contained in envelopes made out of paper and mail articles contained in envelopes made out of plastics material.
To this end, the invention provides a method of distinguishing between mail articles contained in envelopes made out of paper and mail articles contained in envelopes made out of plastics material, the method being characterized in that it consists in displacing each mail article in front of a pressure member so as to produce an acoustic signal that results from a suction effect of the pressure member on one face of said article while it moves in front of said pressure member, and in comparing said acoustic signal with a reference signal in order to detect whether the envelope of said article is made out of paper or out of plastics material. More particularly, according to the invention, spectral characteristics are extracted from said acoustic signal and are compared with corresponding reference values in order to detect whether the envelope of said article is made out of paper or out of plastics material. It has thus been observed that in an installation for unstacking mail articles that is provided with a dual-point separator constituted by a suction nozzle, the noise that results from the suction effect by the dual-point separator on one face of an unstacked article while said article moves in front of the suction nozzle is different depending on whether the mail article is contained in an envelope made out of paper or is contained in an envelope made out of plastics material. More particularly, spectral analysis both of the acoustic signals picked up for mail articles contained in envelopes made out of plastics material and of the acoustic signals picked up for mail articles contained in envelopes made out of paper has shown that the first acoustic signals corresponding to envelopes made out of plastics material all present a characteristic of high amplitude in a certain frequency range, which characteristic is not found in the acoustic signals corresponding to envelopes made out of paper.